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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 108:205–222 (1999)
Kinematic Data on Primate Head and Neck Posture:
Implications for the Evolution of Basicranial Flexion and an
Evaluation of Registration Planes Used in Paleoanthropology
DAVID S. STRAIT1* AND CALLUM F. ROSS2
Program in Anthropological Sciences, State University of New
York at Stony Brook, Stony Brook, New York 11794–4364
2Department of Anatomical Sciences, Health Sciences Center, State
University of New York at Stony Brook, Stony Brook, New York 11794–8081
1Doctoral
KEY WORDS
kinematics; posture; cranial base flexion; Frankfurt
Horizontal; orbital plane
ABSTRACT
Kinematic data on primate head and neck posture were
collected by filming 29 primate species during locomotion. These were used to
test whether head and neck posture are significant influences on basicranial
flexion and whether the Frankfurt plane can legitimately be employed in
paleoanthropological studies. Three kinematic measurements were recorded
as angles relative to the gravity vector, the inclination of the orbital plane, the
inclination of the neck, and the inclination of the Frankfurt plane. A fourth
kinematic measurement was calculated as the angle between the neck and
the orbital plane (the head-neck angle [HNA]). The functional relationships of
basicranial flexion were examined by calculating the correlations and partial
correlations between HNA and craniometric measurements representing
basicranial flexion, orbital kyphosis, and relative brain size (Ross and Ravosa
[1993] Am. J. Phys. Anthropol. 91:305–324).
Significant partial correlations were observed between relative brain size
and basicranial flexion and between HNA and orbital kyphosis. This indicates
that brain size, rather than head and neck posture, is the primary influence
on flexion, while the degree of orbital kyphosis may act to reorient the visual
field in response to variation in head and neck posture. Regarding registration
planes, the Frankfurt plane was found to be horizontal in humans but
inclined in all nonhuman primates. In contrast, nearly all primates (including
humans) oriented their orbits such that they faced anteriorly and slightly
inferiorly. These results suggest that for certain functional craniometric
studies, the orbital plane may be a more suitable registration plane than
Frankfurt ‘‘Horizontal.’’ Am J Phys Anthropol 108:205–222, 1999.
r 1999 Wiley-Liss, Inc.
Humans have a highly flexed cranial base.
This unusual feature has been linked to
other unusual human features, most notably brain enlargement and orthograde posture. Hypotheses relating basicranial form
to brain size date back to the early part of
this century (e.g., Bolk, 1926a,b). The
premise of all brain-size hypotheses is that
because the cranial base is also the floor of
the cranial cavity, brain size is a fundamenr 1999 WILEY-LISS, INC.
tal constraint on basicranial form. As species evolve to have larger brains, the bones
of the cranial base must reorganize to accommodate a larger organ (Moss, 1958; Du Brul
Grant sponsor: NSF; Grant number: SBR9528921.
*Correspondence to: David S. Strait, Department of Anthropology, The George Washington University, 2110 G Street NW,
Washington, DC, 20052. E-mail: [email protected]
Received 22 January 1998; accepted 4 October 1998.
206
D.S. STRAIT AND C.F. ROSS
and Laskin, 1961; Biegert, 1963; Vogel, 1964;
Enlow, 1976, 1990; Riesenfeld, 1969; Gould,
1977). These modifications provide the solution to what Ross and Ravosa (1993:307) call
the ‘‘spatial-packing’’ problem caused by the
brain.
Recently, Ross and Ravosa (1993) demonstrated significant correlations between flexion and a measure of brain volume relative
to basicranial length across a broad sample
of primate species. Ross and Henneberg
(1995) further found that, relative to other
primates, humans have less flexed basicrania than would be expected given their
relative brain size. They suggest that the
cranial base is structurally constrained such
that the angle between the clivus and presphenoid planes cannot fall much below 90°
and cannot exceed 180° (i.e., that the relationship between brain size and flexion may
be logistic rather than linear [more information is available in a new article submitted
by Ross et al.]). Ross and Henneberg (1995)
argue that the lower limit of 90° was attained early in human evolution, as demonstrated by Australopithecus africanus, precluding further flexion in response to
increases in brain size. Subsequent increases in brain size were thereafter accommodated by changes in other aspects of
cranial form (e.g., a high, rounded cranial
vault). Spoor (1997) has recently questioned
whether the brain size–flexion relationship
is logistic, but his disagreement focuses on
only a single species (Homo sapiens). With
respect to other primates, his study corroborates the finding that relative brain size is
related to flexion.
One implication of these conclusions is
that the evolution of orthograde posture had
little to do with the evolution of basicranial
flexion among hominids. This argument was
made by Ross and Ravosa (1993) on the
basis of a simple nonparametric test of the
hypothesis that orthograde primates have
more flexed basicrania than pronograde ones.
However, Ross and Ravosa’s (1993) study
suffered from a lack of quantitative data on
head and neck posture. They defined orthograde posture qualitatively such that orthograde species were those that habitually
employed orthograde body postures while
sitting, standing, or locomoting (e.g., leap-
ers, brachiators). This behavioral classification is imprecise because body posture (the
orientation of the trunk relative to gravity)
is not directly relevant to basicranial morphology, insofar as the trunk does not articulate with the skull. Consequently, animals
with orthograde body posture can nonetheless display pronograde neck posture (e.g.,
indriids). Rather, when discussing the postural hypothesis, the relevant measure of
posture is either neck posture (the inclination of the neck relative to gravity) or the
orientation of the head relative to the neck.
Although some data on primate head and
neck posture exist (Vidal et al., 1986; Graf et
al., 1995a,b), they cannot easily be applied
to studies of comparative primate anatomy
because they have been collected on only a
few species (Macaca fascicularis, M. mulatta, Saimiri sciureus, Cebus apella, Homo
sapiens). The limited taxonomic scope of
these studies reflects the fact that they
employed methodologies (radiography, cineradiography) that can be applied only under
laboratory conditions. Moreover, these studies did not collect data on primates during
locomotor behaviors. Given that body movements during locomotion tend to be highly
repetitive and that the entire skeleton may
experience higher loads during locomotion
than at rest, it is reasonable to suspect that
head and neck postures during locomotion
may be particularly relevant to studies relating posture to cranial form.
The present study aimed to rectify these
deficiencies by collecting quantitative data
on head and neck posture in living primates
and using this data to test the relationships
between relative brain size, head and neck
posture, and cranial base flexion. We employed methods that, while necessarily less
precise than those used by Vidal et al. (1986)
and Graf et al. (1995a,b), allowed a greater
sampling of primate species as well as the
collection of postural data during locomotion.
In addition, this study sought to evaluate
registration planes used in paleoanthropological analyses. The Frankfurt plane is
widely used in such studies, principally because humans are known to hold this plane
in a roughly horizontal orientation during
habitual postures (Downs, 1952). Although
HEAD AND NECK POSTURE
207
Fig. 1. Relationships between head posture, neck posture, basicranial flexion, and orbit frontation.
A: Primate with pronograde neck posture, an anteriorly directed visual field, and an unflexed base.
B: Primate in orthograde neck posture but without morphological alterations allowing it to maintain an
anteriorly directed visual field. Three possible alterations include a flexed base (C), kyphotic orbits (D),
and a flexed base and kyphotic orbits (E). Figure after Ross (1995).
this plane was designed to be used only in
studies of humans (e.g., Enlow, 1990; Merow
and Broadbent, 1990), it has been extensively applied to studies of early hominids
(e.g., Tobias, 1967, 1991; Kimbel et al., 1984;
Kimbel and White, 1988; Aiello and Dean,
1990; Luboga and Wood, 1990; Wood, 1991;
Grine et al., 1993, 1996; Strait et al., 1997).
Moreover, insofar as such studies employ
measurements of nonhuman primates, it
has been applied to those species as well.
Even nonpaleontological studies have used
the Frankfurt plane to characterize primate
head posture (Vidal et al., 1986; Graf et al.,
1995a,b). The influence of Frankfurt Horizontal is so pervasive that it affects morphological interpretations in studies that do not
even name it explicitly. For instance, whenever a relatively complete fossil hominid
skull is described, it is conventional to depict
it in the Frankfurt plane and to characterize
its morphology accordingly (e.g., Walker et
al., 1986; Kimbel et al., 1994). Unless fossil
hominids and nonhuman primates hold this
plane in a horizontal orientation (like humans), the data collected using this plane
will be arbitrary and thus of questionable
significance.
HYPOTHESES
Basicranial flexion
The premise of the postural hypothesis is
that as primate species evolve from being
pronograde (Fig. 1A) to being orthograde
(Fig. 1B), they must maintain an anteriorly
directed visual field. There are several ways
to achieve this. The first is simply to have a
208
D.S. STRAIT AND C.F. ROSS
very flexible neck. This is an appropriate
null hypothesis for theories attempting to
relate posture to cranial morphology, for, if
postural changes are not accompanied by
morphological adaptations of the skull, they
must be accommodated via morphological
changes or behavioral adaptations in the
neck.
Another solution is to flex the cranial base
as the neck becomes upright, thereby maintaining the orbits in a roughly vertical orientation (Wood Jones, 1917; Weidenreich, 1924,
1941; Ross, 1995) (Fig. 1C). According to
Weidenreich (1941:424),
the base of the human skull is deflected at the junction of
pre- and postsphenoid so that the anterior part of the
base is bent downwards. This skull ‘‘kyphosis’’ counterbalances the ‘‘lordosis’’ of the cervical portion of the
spine. It brings the face down in adaptation to the new
direction of locomotion which otherwise would have
been looking upwards had it preserved its original
orientation to the spine.
Under this hypothesis, flexion is dependent
on and should be correlated with the orientation of the orbits relative to the neck.
Dabelow (1929) suggested an alternative
solution in which the orbits of primates
become kyphotic, meaning that the orbits
are rotated ventrally on the skull (Fig. 1D).
According to this hypothesis, orbital kyphosis, not basicranial flexion, should be correlated with an anteriorly directed visual field
and an orthograde neck.
Dabelow (1929) also suggested that that
both flexion and orbital kyphosis will occur
in those species with highly approximated
orbits (presumably because the anterior cranial base and upper face are structurally
contiguous in these species). This hypothesis predicts that both flexion and frontation
will be correlated with head and neck posture in species such as haplorhines and in
particular cercopithecines (Fig. 1E).
The plausibility of the postural hypothesis
is strengthened by recent observations that
primates exhibit a restricted range of motion at the atlanto-occipital joint (Graf et al.,
1995a). This suggests that primates cannot
compensate for the inclination of the neck
merely by reorienting their heads, perhaps
necessitating some form of a morphological
adjustment.
Registration planes
We also tested the validity of the Frankfurt Horizontal as a reference plane for
craniometric research into morphological correlates of postural differences. Obviously, all
registration planes exert considerable influence on the results of a morphological analysis, and for that reason some researchers
(for review see Cole, 1996; Moss et al., 1987)
have turned to methods that do not require
such a plane (e.g., Euclidean distance matrix analysis [Lele, 1991, 1993; Lele and
Richtsmeier, 1991, 1995; Lele and Cole, 1995,
1996]). However, certain research questions
can be addressed only by employing a registration plane. For instance, the biomechanical significance of certain features (e.g., the
effect of nuchal plane inclination on the
mechanical advantage of the nuchal muscles)
can be understood only in reference to the
gravity vector, because the weight of an
object or the torque imposed by its center of
mass may be important biomechanical parameters. For such studies, a registration
plane that exhibits a fairly consistent angular relationship with the gravity vector is
desirable.
In general, a registration plane can be
used if it exhibits a functional, structural, or
developmental property that is consistent
across all of the taxa being studied. If this
property is not constant across taxa, the
resulting data will be tainted with a registration artifact, which is essentially a bias
introduced by the plane that does not reflect
the functional or structural parameter for
which the measurement was designed. In
the case of the Frankfurt plane, the property
in question is head posture, as indicated by
the fact that many studies of the biomechanics and physiology of head balance have
employed the Frankfurt plane (e.g., Schultz,
1942; Ashton and Zuckerman, 1951, 1952,
1956; Adams and Moore, 1975; Luboga and
Wood, 1990; Graf et al., 1995a,b). Because
nonhuman primates are often used as comparative samples with which to interpret
human morphology, this plane would be
suitable for representing habitual head posture if it was oriented similarly in human
and nonhuman primates. If so, then presumably the Frankfurt plane would likewise
HEAD AND NECK POSTURE
TABLE 1. Summary statistics for kinematic
Species
Eulemur fulvus
Lemur catta
Varecia variegata
Alouatta seniculus
Ateles fusciceps
Ateles geoffroyi
Cebus apella
Lagothrix lagotricha
Saguinas mystax
Saimiri sciureus
Cercopithecus aethiops
Cercopithecus albogularis
Cercopithecus diana
Cercopithecus mona
Cercopithecus petaurista
Erythrocebus patas
Macaca fascicularis
Macaca fuscata
Macaca mulatta
Papio hamadryas
Papio ursinus
Colobus angolensis
Colobus guereza
Hylobates lar
Symphalangus syndactylus
Pongo pygmaeus
Gorilla gorilla
Pan troglodytes
Homo sapiens
Data
collection
condition
209
measurements1
Inclination
Frankfurt
plane
Substrate
Inclination
orbital plane
Inclination
neck
Mode
N
Mean
SD
N
Mean
SD
N
4.0
5.1
7.7
7.2
4.7
10.6
10.6
15.8
6.4
5.8
6.3
5.8
4.9
8.0
7.6
9.2
10.4
9.5
9.5
8.5
5.3
10.7
9.4
9.6
7.8
6.9
6.7
6.1
9.7
4.8
11
88.3
9.0
28
88.7
9.5
13
91.7 10.6
33 107.6
6.9
25
75.1
7.3
10
62.8
6.6
10
62.8
6.6
11
50.5
9.2
38
61.1
6.2
29
89.0
8.2
24
63.1
8.6
22
70.9
7.5
40
48.5 12.0
27
58.7
6.6
39
90.0
8.2
39
73.1
9.7
38
70.4
5.7
43
70.3
8.9
26
53.6
5.1
25
55.1
6.4
34
62.0
5.8
20
51.8
8.1
27
57.2
8.1
20
58.4
6.1
18
61.9
5.6
21
70.1 10.4
18
71
11.3
23
89.9
6.4
53
47.0
9.7
2
49.7
3.7
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
C
C
S
S
S
S
U
S
S
S
S
U
W beam
Ground
W beam
W beam
Cage flr
Ir beam
Ground
Ir beam
W log
Cage flr
Ir beam
Cage flr
W log
W beam
W beam
Ir beam
W beam
W beam
Ground
W beam
W beam
Ground
W beam
Cage flr
Ground
W beam
Ir beam
Ir beam
Ir beam
W beam
Quad
Quad
Quad
Quad
Quad
Brach
Quad
Brach
Quad
Quad
Brach
Quad
Quad
Quad
Quad
Quad
Quad
Quad
Quad
Quad
Quad
Quad
Quad
Quad
Quad
Quad
Quad
Leap
Brach
Brach
3
26
4
29
2
21
6
8
56
21
22
10
42
19
2
31
26
44
26
23
36
25
33
13
17
7
17
23
59
9
52.9
61.4
64.5
54.6
63.3
66.6
67.7
74.9
77.0
65.4
79.8
73.2
75.6
68.2
57.9
56.2
55.4
62.9
72.9
67.0
70.6
84.6
61.9
60.5
71.0
49.8
55.7
49.1
82.1
80.6
16.6
7.4
4.0
10.9
1.2
13.7
6.3
10.9
8.2
7.6
9.6
6.6
4.8
9.9
0.9
11.2
9.8
8.0
10.7
6.8
6.0
11.7
10.3
18.0
8.2
12.8
9.5
5.6
13.9
10.6
3
17.3
24
5.0
6
6.2
26
21.0
19
11.9
28
11.4
10
10.9
19
1.5
52
13.1
21
18.5
29
11.3
9
11.0
41
10.8
24
17.8
18
23.9
24
16.1
29
22.6
40
18.5
26
12.3
23
25.2
35
10.9
27
2.1
33
16.9
22
19.6
16
11.2
18
19.5
16
25.7
17
27.7
40
2.0
10 ⫺11.6
U
S
U
C
C beam/rope
Ground
Ground
Ground
Susp
Knuck
Knuck
Biped
24
20
45
28
75.0
59.6
49.2
93.2
28.4
11.2
12.2
3.1
43
31
47
28
17.1 37.2 47
18.4 5.3 33
23.4 7.8 54
9.3 5.7 44
Mean
SD
55.0 28.9
56.4
6.0
81.5
8.0
17.9
7.3
1 Data collection conditions: C, controlled; S, semicontrolled; U, uncontrolled. Substrates: C beam, cement beam; Cage flr, cage floor; Ir
beam, iron beam; W beam, wooden beam; W log, very thick wooden beam. Mode of locomotion: Biped, bipedalism; Brach, brachiation;
Knuck, knuckle-walking; Leap, quadrupedal leaping; Quad, quadrupedalism; Susp, quadrumanous suspension.
have been similarly oriented in fossil
hominids. But if humans and nonhuman
primates differ, then the possibility exists
that fossil hominids might not have oriented
the plane in a human-like fashion.
MATERIALS AND METHODS
Subjects
To test these hypotheses, we collected
data on head and neck posture from adult
representatives of 29 species of primates
(Table 1), took measures of basicranial flexion and orbital kyphosis from lateral radiographs of skulls representing these same 29
species, and also took a measure of relative
endocranial volume from the same skulls.
Postural measures
Three kinematic measures of head and
neck posture were taken from video images
of primates filmed during locomotion: the
inclination of the neck, the inclination of the
orbital plane, and the inclination of the
Frankfurt plane. The inclination of the orbital plane was measured as the angle relative to the gravity vector of the line joining
the superior and inferior margins of the
orbits. Neck posture was measured as the
inclination of the dorsal surface of the neck
relative to gravity. A similar measurement
was found to be significantly (but weakly)
correlated with the inclination of the cervical vertebral bodies in an intraspecific sam-
210
D.S. STRAIT AND C.F. ROSS
Fig. 2. Basicranial flexion and the interaction between head and neck posture. A: Primate in orthograde
posture with an anteriorly directed visual field.
B: Primate in more pronograde posture with an inferiorly directed visual field. In both A and B, the angular
relationship between head and neck posture is equivalent, resulting in equivalent degrees of basicranial flexion.
ple of humans (Refshauge et al., 1994).
Because the inclination of the cervical vertebrae was not measured directly, the neck
posture data presented here should be considered approximate. In order to test the
validity of the Frankfurt ‘‘Horizontal’’ as a
measure of head posture, the inclination of
the line infraorbitale–porion relative to gravity was measured.
A fourth measure of posture was calculated that reflects the orientation of the
head relative to the neck. This final measurement is the critical one needed to evaluate
hypotheses relating differences in posture to
differences in basicranial flexion. Consider a
case in which a primate species habitually
holds its orbits in a nearly vertical orientation and its neck in orthograde posture (Fig.
2A). Because the basicranium is a structural
interface between the neck and head, this
species is expected to display, among other
features, an internally flexed cranial base.
Now consider a species that has a more
pronograde neck posture but that habitually
inclines its orbits so they face more inferiorly (Fig. 2B). Despite the fact that the latter
species has a more horizontally inclined
neck, both species exhibit an equivalent
degree of basicranial flexion. The reason for
this is that the relative orientations of the
orbits and neck in both species are equivalent. Thus, the relative inclinations of the
orbital aperture and the neck must be measured. Such a measure, the head-neck angle
(HNA), was obtained by subtracting the
mean value for orbit inclination from the
mean value for neck inclination for each
species. Ideally, HNA would be calculated by
subtracting orbit inclination from neck inclination for each video image that was digitized, but this was impractical because it
was often the case that only one of the two
measures could be taken on a given video
image.
Postural data were collected by filming
living primates during locomotion and then
digitizing the images on a computer and
extracting angular measurements using
MacMorph software (Spencer and Spencer,
1993, 1995). Our data do not capture the full
range of head and neck movement. In particular, no attempt was made to collect data
when subjects were at rest, as preliminary
observations suggested that resting animals
hold their heads and necks in a wide variety
of positions. Although resting postures may
be significant influences on basicranial form,
it is noteworthy that subjects employed a
much more restricted repertoire of head and
neck postures during locomotion, suggesting
that head and neck posture during locomotion may be under stricter selective control
than during resting.
Postural data were collected under three
types of conditions: controlled or laboratory
conditions, semicontrolled or zoo conditions,
and uncontrolled or wild conditions. Under
controlled conditions, subjects were shaved,
and colored cotton markers were affixed
(glued) to points on the skin overlying craniometric and cervical landmarks (inion, glabella, infraorbitale, supraorbitale, porion,
spinous process C2, spinous process C7).
Subjects were then coaxed, using food rewards, to move back and forth along a
known path. As they moved, the primates
were simultaneously filmed in lateral and
superior view. A laterally placed (high resolution) camera was positioned such that it was
perpendicular to the expected path of the
subject, and it was this camera that provided video images from which kinematic
data were collected. A plumb bob was suspended in the background to indicate the
gravity vector. Frames in which the subject’s
forelimb was in either midstance or midswing were selected for digitization (midswing refers to the middle of the swing cycle
HEAD AND NECK POSTURE
of suspensory locomotion, not the swing
phase of a step cycle). However, suitable
kinematic data could be obtained only when
the magnification of the camera was set
such that the head and neck filled the screen.
Thus, the timing of midstance and midswing
could only be approximated.
A superiorly placed (standard resolution)
camera was positioned above the expected
path of the subject and was used to select
frames in which the head and neck were
perpendicular to the main camera. Perpendicular was defined as the state in which the
long axes of the head (glabella–inion) or
neck (spinous processes of C2–C7) were
within 15° of the line indicating the expected
path of the subject. Note that C2 and C7
were not used as landmarks for the collection of neck inclination data but were used
only to assess the deviation of the neck from
the axis perpendicular to the laterally placed
camera. Neck inclination was measured
along the flattest surface of the dorsal aspect
of the neck. This was necessary in order to
make the data collected under controlled
conditions comparable to those collected under other conditions and because primate
species exhibited variability as to the inclinations of different portions of their necks.
Qualitatively, neck inclination was most often measured between approximately C2
and C5.
Under semicontrolled conditions, subjects
were neither shaved nor marked. Hair was
an important confounding factor, but most of
the species filmed under these conditions
had neck hair that was short or of only
moderate length. Filming conditions varied
somewhat among species because of the
unique constraints imposed by the cages or
enclosures in which each species was filmed.
In general, subjects were filmed with a
laterally placed camera, and a leveling tool
was used to ensure that the camera was
mounted in the horizontal plane. Thus, the
vertical axis of the screen was used to approximate the gravity vector. As before, subjects were filmed simultaneously in two
views, but, instead of being filmed from
above, they were filmed with a camera that
was positioned along the axis of their expected path (i.e., a camera that filmed sub-
211
jects from either the front or the back,
depending on the direction in which the
subject was moving). The axial camera
served the same purpose as the superiorly
placed camera, to select frames for digitization, but the selection process was qualitative rather than quantitative.
Under uncontrolled conditions, subjects
were neither shaved nor marked, and they
were filmed from an approximately lateral
position. Subjects were not coaxed using
food rewards, and thus they were filmed
opportunistically. Only one camera was used
to film the primates, and it was held in an
approximately horizontal orientation.
Most species were filmed during quadrupedal locomotion, although some were filmed
during brachiation, quadrupedal leaping,
quadrumanous suspension, knuckle-walking, and bipedalism. Four species engaged
in two modes of locomotion (A. geoffroyi, A.
fusciceps, L. lagotricha, C. guereza). Quadrupeds, bipeds, and knuckle-walkers were
filmed during walking gaits. Brachiators
were filmed during slow brachiation (i.e.,
the subject always had at least one limb
grasping a support). Quadrupedal leaps were
of short distance and between horizontal
supports. Certainly faster or more dynamic
gaits may be important selective factors
affecting posture and cranial morphology,
but the enclosures in which most of the
subjects were filmed generally did not allow
such gaits; by the time a subject accelerated
to a running gait, it would exhaust the
available space and have to slow down. The
number of observations for each measurement varied widely among and sometimes
within species, largely due to the varying
conditions under which the species were
filmed (e.g., lighting, configuration of cages),
the degree to which the subjects cooperated
with the experiment, and the clarity of the
video images. With respect to most species,
the subjects were separated from the cameras by a chain-link cage wall, which often
had the effect of obscuring cranial or cervical
landmarks on the video images. As a result,
the final sample sizes represent only a fraction (about 20–50%) of the number of times
that a subject passed in front of the laterally
placed camera.
D.S. STRAIT AND C.F. ROSS
212
Craniometric measures
Data on the degree of basicranial flexion,
orbital kyphosis, and relative brain size
were obtained for the same 29 species for
which postural data were gathered. The
degree of basicranial flexion was measured
using the cranial base angle (CBA), which is
calculated as the angle between the clivus
and presphenoid planes. This measure sums
the pre- and postsella flexions observed in
primates and other mammals (Moss and
Vilman, 1978). Note that flexion increases as
the angle decreases. The degree of orbital
kyphosis was measured using the angle of
orbital axis orientation (AOA), calculated as
the angle between the clivus and the axis
passing from the optic canal through the
center of the orbits. Relative brain size was
measured using the index of relative encephalization (IRE), calculated as the cubed
root of neurocranial volume divided by basicranial length (BL). BL reflects a series of
chord distances taken along the midline
endocranial contour of the basioccipital, basisphenoid, and presphenoid. Formal definitions of these measurements can be found in
Ross and Ravosa (1993), and most of these
data were taken from Ross (1993), Ross and
Ravosa (1993), and Ross and Henneberg
(1995). Additional data were gathered for
species not represented in those studies.
It is worth emphasizing the distinction
between two types of orbital orientation
measurements. Orbit inclination refers to a
kinematic measure of head posture, specifically the direction in which the orbits face
when the head is held in habitual postures.
Orbital kyphosis is a craniometric measure
of how the orbits are positioned within the
skull (Ross and Ravosa, 1993).
Statistical analyses
The hypotheses were evaluated by computing Pearson’s product-moment correlations
(P ⬍ 0.05) for all bivariate comparisons
between HNA, IRE, CBA, and AOA. In
addition, partial correlation analyses (P ⬍
0.05) were carried out to isolate that portion
of the total variance in CBA associated with
one variable while controlling for correlations with other independent variables. In
all bivariate comparisons, humans were out-
liers. Because outliers can have an undue
influence on statistical tests, correlations
and partial correlations were calculated on
two data sets, one that included humans and
one that consisted only of nonhuman primates.
The validity of the Frankfurt plane was
evaluated by determining 1) whether it
showed unusually high variability as measured by its standard deviation, 2) whether
human and nonhuman primates exhibited
similar values, as evaluated by examination
of a histogram of values for all primates
studied, and 3) whether the relative orientations of the Frankfurt and orbital planes
indicate that the former might have been
held horizontally in fossil hominids.
RESULTS
It is worth reiterating that the three primary kinematic measurements are angles
taken relative to gravity, but they are not
read in the same manner. The Frankfurt
plane is horizontal when it is oriented at 90°,
and it slopes anteriorly and inferiorly when
inclined at less than 90°. The orbital plane is
vertical when it is oriented at 0°, and the
obits face anteriorly and inferiorly as the
angle increases in value. Negative values
indicate that the species looks somewhat
superiorly. The neck is horizontal when it is
oriented at 90° and becomes more vertically
inclined as the angle approaches 0°.
Head posture
The head posture data presented in Table
1 are summarized in Figure 3, which shows
the distribution of species means for orbit
inclination and the inclination of the Frankfurt plane. For the purposes of this figure,
head posture values for species that engaged
in two modes of locomotion were calculated
as the average of the mean for each locomotor mode. Regarding orbit inclination, the
species clustered into a tight, peaked distribution whose mean is 13.7°. In other words,
most primate species hold their heads such
that their orbits face anteriorly and slightly
inferiorly, as indicated by the fact that the
values for these species are at or slightly
above zero. Symphalangus syndactylus is
the only species that looks superiorly at the
HEAD AND NECK POSTURE
213
Fig. 3. Histogram of the distribution of species means
for head-posture measurements. White vertical bars
indicate species means for orbit inclination. Grey vertical bars indicate species means for the inclination of the
Frankfurt plane. Black vertical bars indicate the values
of Homo sapiens for the two measurements. An orbit
inclination of 0° indicates that the orbits are vertical.
The Frankfurt plane is horizontal when inclined at 90°.
Solid vertical lines indicate the orbit inclination of fossil
hominid specimens, assuming that the Frankfurt plane
is oriented as in humans. Dashed vertical lines indicate
the inclination of the Frankfurt plane in hominids,
assuming that the orbital plane is oriented as in humans.
time our head orientation measures were
taken.
The data on the Frankfurt plane indicate
that, in most primates, this plane is not held
horizontally during locomotion but is inclined such that infraorbitale is inferior to
porion. Moreover, humans are outliers to the
rest of primates, being the only species in
which the plane is held horizontally.
The standard deviations of mean orbit
inclination and mean Frankfurt plane inclination are 8.0 and 11.2, respectively. This is
consistent with the observation (Fig. 3) that
the Frankfurt plane distribution is flatter
than the distribution of orbital plane values
and appears bimodal. Note that the coefficient of variation (CV) was not employed as
a measure of variability because the mean of
an angle (and thus its CV) varies according
to its frame of reference. For instance, an
angle of 160° is logically equivalent to one of
20°, but the means are very different.
TABLE 2. Angular difference between the orbital
and Frankfurt planes in fossil hominids
Specimen
Difference (in degrees)
Sts 5
KNM-WT 17000
OH 5
KNM-ER 406
KNM-ER 1813
Stw 53
KNM-ER 3733
96.4
66.2
85.6
83.7
83.7
87.2
104.1
Head posture in early hominids cannot be
directly observed, but it is possible to measure the angulation between the Frankfurt
and orbital planes on fossil crania (Table 2).
If it is assumed that all fossil hominids held
their heads such that the orientation of the
Frankfurt plane was the same as in humans, then many fossil hominids would
have held their orbits such that they faced
superiorly (Fig. 3). In contrast, if it is assumed that early hominids had a human-
D.S. STRAIT AND C.F. ROSS
214
Fig. 4.
Histograms of mean neck inclination in primates divided according to locomotor mode.
like value for orbit inclination, then the
orientation of the Frankfurt plane falls easily within the range of living primates
(Fig. 3).
Neck posture
It is obvious from Table 1 that quantitative kinematic data on neck inclination represent a vast improvement over qualitative
generalizations such as pronograde and orthograde (Fig. 4). For instance, it would be
misleading to characterize brachiators as
having orthograde neck posture because
necks in these species are actually fairly
strongly inclined. Moreover, although quadrupeds are the only species to exhibit horizontally inclined necks, the range of neck
inclination in quadrupedal species (48.5–
107.6°) almost encompasses that of the brachiating species (47.0–63.1°). The best example of the inadequacy of using locomotor
mode as a surrogate for neck posture is
provided by the African apes. Given that
Pan and Gorilla both employ a unique form
of locomotion (knuckle-walking), one would
expect them to have similar values for neck
inclination. In fact, these species differ on
average by about 25°.
This is not to say that neck inclination is
unaffected by locomotor mode. For instance,
t-tests revealed that within L. lagotricha
and A. fusciceps brachiation was associated
with significantly more vertical neck posture than quadrupedalism (P ⬍ .0001). The
same relationship was nearly significant in
A. geoffroyi (P ⫽ .057), but the comparison
was hindered by small sample size. Within
C. guereza, neck posture was significantly
more horizontal during leaping than during
quadrupedal walking (P ⬍ .0001). Of course,
intraspecific comparisons may not be directly comparable to interspecific ones; selection does not act to transform the cranial
morphology of a spider monkey as it switches
from brachiation to quadrupedalism. Thus,
although intraspecific comparisons reveal
that neck inclination is influenced by locomotor mode, interspecific comparisons indicate
that this influence is not so strong as to
support broad, qualitative generalizations.
Correlations
Pearson correlation coefficients for bivariate comparisons of CBA, AOA, IRE, and the
HNA are shown in Table 3. All four variables
are significantly correlated with each other
HEAD AND NECK POSTURE
TABLE 3. Bivariate correlations between flexion,
orbital kyphosis, relative brain size and posture
CBA
AOA
IRE
Correlations among all species
CBA
1.00
AOA
.74*
1.00
IRE
⫺.83*
⫺.71*
1.00
HNA
.76*
.81*
⫺.71*
Correlations among nonhuman primates
CBA
1.00
AOA
.54*
1.00
IRE
⫺.68*
⫺.38
1.00
HNA
.65*
.73*
⫺.60*
HNA
1.00
1.00
* P ⬍ .05.
in the all-species sample, as they are in the
nonhuman primates sample, with the exception of AOA and IRE. Bivariate plots of HNA
against the other three variables are shown
in Figure 5, and these reiterate the degree to
which humans are outliers. Similar plots
between CBA, AOA, and IRE are presented
in Ross and Ravosa (1993) and Ross and
Henneberg (1995).
Partial correlation coefficients are shown
in Table 4. In both samples, when variation
in other variables is accounted for, basicranial flexion (CBA) is found to be significantly
correlated with relative brain size (IRE),
and the head-neck angle (HNA) is found to
be correlated with orbital frontation (estimated by AOA). Relative brain size and
posture are not significantly related to each
other when AOA and CBA are taken into
account.
DISCUSSION
A number of unavoidable factors may have
affected the kinematic data. First, due to a
dearth of subjects or because dominant individuals in a group monopolized the food
rewards, there were usually only one or a
few individuals filmed per species. Thus,
interindividual variation is not well documented by our data set. Second, the substrates varied according to the conditions
under which the subjects were filmed (Table
1). Substrates may have an important effect
on head posture because an irregular or
arboreal substrate might encourage a primate to focus attention on it. Terrestrial
substrates included the ground and the floor
of a cage. A cage floor contains gaps, and
thus is perhaps a somewhat more irregular
215
substrate than the ground. Arboreal substrates were all horizontal and included iron
bars, wooden branches, and wooden beams
of varying diameters. When possible, subjects were filmed on substrates that matched
their habitual mode of locomotion (i.e., arboreal quadrupeds on arboreal substrates),
but this could not be achieved for every
species. Inclined substrates tended not to be
available. Future work should determine
whether substrate inclination has a strong
effect on head and neck posture. Third,
posture (particularly head posture) could
have been influenced by the fact that under
controlled and semicontrolled conditions primates were coaxed to move in front of the
cameras with food rewards. Qualitatively,
such a bias did not appear to be common;
most often, subjects saw the food reward but
then adjusted their head posture during
locomotion so they were not looking directly
at it. Finally, the neck posture measurement
may be unduly influenced by the soft tissue
structures of the neck, particularly the nuchal muscles. Refshauge et al. (1994) found
that the mean inclinations of the dorsal
surface of the neck and the cervical vertebrae differed by only 4° in a sample of
humans, but this discrepancy is likely to be
greater in species with greater nuchal muscle
mass, such as G. gorilla.
For these reasons and because most species were filmed under semicontrolled conditions, the data presented here should be
considered approximate. Nonetheless, these
data are still considerably more accurate
than the qualitative characterizations (pronograde or orthograde) employed by Ross
and Ravosa (1993).
Head posture
Most primate species were found to hold
their heads with their orbits facing anteriorly and slightly inferiorly. The exception to
this generalization is Symphalangus syndactylus, which looked superiorly at the time
our head posture measures were taken. This
occurred because siamangs must look up
during brachiation to locate their next handhold. In fact, all brachiators change their
head posture throughout the swing cycle, so
at the beginning of the cycle the orbits face
slightly inferiorly and at the end of the cycle
D.S. STRAIT AND C.F. ROSS
216
Fig. 5.
Bivariate scatterplots of HNA vs. CBA (A), AOA (B), and IRE (C).
they face somewhat superiorly. At approximately midswing (when orbit inclination
was measured), all species raise their heads.
The value for orbit inclination in S. syndacty-
lus is exceptional compared to other brachiators because this species tended to raise its
head earlier in the swing cycle (just before
midswing rather than at or after midswing).
HEAD AND NECK POSTURE
TABLE 4. Partial correlations between flexion, orbital
kyphosis, relative brain size and posture
CBA
AOA
IRE
HNA
Partial correlations among all species
CBA
1.00
AOA
.15
1.00
IRE
⫺.59*
⫺.17
1.00
HNA
.29
.55*
⫺.08
1.00
Partial correlations among nonhuman primates
CBA
1.00
AOA
.19
1.00
IRE
⫺.49*
⫺.18
1.00
HNA
.21
.61*
⫺.33
1.00
* P ⬍ .05.
Because head posture changes during the
locomotor cycle in these species, the measurements recorded here represent gross simplifications of the actual head postures employed by them. Consequently, the postural
data for these species must be viewed cautiously. In most other species, head posture
remained qualitatively consistent throughout a cycle, but clearly a quantitative study
of intracycle variability in head posture is
warranted.
In general, however, orbit inclination data
confirmed the notion that primates maintain an anteriorly directed visual field. In
reality, the visual field faced somewhat inferiorly, consistent with the fact that most
primate species utilize an inferiorly placed
substrate. Accordingly, the premise of the
postural hypothesis was supported.
The observation that all primates maintain an anteriorly directed visual field raises
the possibility that the orbital plane may be
more suitable than the Frankfurt plane for
use as a registration plane in craniometric
studies aimed at studying morphological
correlates of posture. Although the orbit
inclination measure shows lower variability
than the Frankfurt plane, we do not feel that
this difference alone precludes the Frankfurt plane from use as a registration plane.
However, when one examines the values for
human and nonhuman primates, a different
picture is found: humans are well within the
primate range for orbit inclination, but they
are outside the range of nonhuman primates
for Frankfurt plane orientation (Fig. 3).
Humans are the only species in which the
Frankfurt plane is truly horizontal; the plane
is inclined in every other primate species,
and in some cases the inclination is quite
steep.
217
This result has implications for functional
craniometric studies, as can be demonstrated by considering the foramen magnum. There has been considerable interest
in how the foramen magnum is oriented and
positioned during habitual postures in human and nonhuman primates (e.g., Dart,
1925; Senyurek, 1938; Schultz, 1942, 1955;
DuBrul, 1950; Ashton and Zuckerman, 1951,
1952, 1956; Le Gros Clark, 1955; DuBrul
and Laskin, 1961; Moore et al. 1973; Adams
and Moore, 1975; Demes, 1985, 1986; Luboga and Wood, 1990). If one measures the
orientation of the foramen relative to Frankfurt Horizontal, as most of the above studies
have done, then the data for human and
nonhuman primates will not be comparable
because the orientation of Frankfurt Horizontal itself differs between humans and
other species. In contrast, humans and nonhuman primates have similar values for
orbit inclination during locomotion, suggesting that studies attempting to determine the
influence of posture on the orientation and
position of the foramen magnum should
instead use the orbital plane, rather than
the Frankfurt Horizontal.
Further evidence on the utility of the
orbital plane (and the inapplicability of the
Frankfurt plane) comes from a consideration of head posture in fossil hominids. As
noted, when certain specimens are oriented
in Frankfurt Horizontal, then they are positioned such that their orbits face superiorly
(Fig. 3). Such an orientation differs from
that of all but one extant primate species
and is unreasonable given that bipedal
hominids have an inferiorly placed substrate (i.e., the ground). However, when
hominid crania are oriented such that their
orbits are inclined as in humans, then their
Frankfurt planes are inclined as in other
primates. In summary, the orbital plane
seems much more likely than the Frankfurt
plane to reflect habitual head posture across
a broad sample of primates and early
hominids. It should be noted, however, that
in H. ergaster and presumably later hominids
the Frankfurt plane was probably inclined
to a degree similar to that seen in modern
humans. Thus, the Frankfurt plane may be
suitable for use in some studies concerning
the later aspects of human evolution.
218
D.S. STRAIT AND C.F. ROSS
It should be noted, of course, that other
planes of head posture have been applied to
the study of human and primate evolution.
In particular, several studies (e.g., Delattre
and Fenart, 1960, 1963) have used the vestibular plane under the assumption that the
lateral semicircular canals are oriented horizontally in primates and other vertebrates
(Girard, 1911, 1923). This is a common
assumption that has yet to be functionally
explained and for which there is little empirical support. The semicircular canals do not
orient the head in space, this being the job of
the otolith organs, the saccule and the utricle
(Kandel et al., 1991; Graf et al., 1995a). The
semicircular canals are instead used to register angular acceleration in three dimensions. In order to accomplish this, the canals
must be oriented perpendicular to each other,
but there is no obvious reason any of the
canals needs to have a particular orientation relative to gravity. It is therefore not
surprising that recent studies have found
that the canals are inclined during resting
postures and locomotion in some mammals
(Vidal et al., 1986; Graf et al., 1995a,b),
although there is some evidence that the
canals may be brought into earth-horizontal
during ‘‘alert’’ postures (Graf et al., 1995b)
Nevertheless, the semicircular canals do
play a role in postural control and coordination of body movements as well as coordinating eye and head movements through their
neuronal connections to the extraoccular
motor system (e.g., Kandel et al., 1991; Graf
et al., 1995a,b). Moreover, although they are
not oriented horizontally, the lateral semicircular canals do display a relatively limited
range of orientations relative to gravity in
resting postures. Our data on orbital plane
orientation suggests a possible explanation
for this regularity. Functional links between
the lateral semicircular canals and the medial and lateral rectus muscles mediate the
horizontal vestibulo-occular reflex, whereby
each lateral semicircular canal excites contractions in its ipsilateral medial rectus and
contralateral lateral rectus muscles. This
reflex ensures that head movements which
stimulate the lateral canals are accompanied by compensatory movements of the eye,
facilitating fixation of objects during relative movements of the observer. The phyloge-
netically conservative nature of this functional system may mean that the lateral
semicircular canals and the medial and lateral rectus muscles are maintained in
roughly the same plane in different species.
Consequently, when the extraoccular muscles are oriented horizontally, so will be the
lateral semicircular canals. It seems reasonable to hypothesize that the medial and
lateral rectus muscles are oriented approximately perpendicular to the orbital plane
measured here, suggesting that they and
the lateral semicircular canals have a relatively constant orientation vs. the gravity
vector purely because of the relatively constant orientation of the orbital margin.
A full evaluation of the vestibular plane
and its applicability to paleoanthropology as
a registration plane would require knowledge of how this plane is oriented during
locomotion in a variety of primate species
(de Beer, 1947; Bull, 1969). Head posture
data collected in the present study should
facilitate such an evaluation; if the inclination of the vestibular plane relative to the
Frankfurt or orbital planes is collected in
the species examined here, then the postural data presented above will allow the
calculation of the orientation of the vestibular plane relative to the gravity vector during locomotor activities.
Determinants of flexion
Bivariate comparisons reveal all four variables examined here (CBA, AOA, IRE, HNA)
to be significantly correlated with each other.
However, partial correlation analyses provide a more subtle picture of the patterns of
correlation between these variables. When
the variability in relative brain size (IRE) is
taken into account, neither posture (HNA)
nor orbital kyphosis (AOA) retains a significant correlation with flexion (CBA). It would
appear that the correlations between flexion
and both HNA and AOA in the bivariate
comparisons are being mediated by correlations between these variables and IRE. This
suggests that relative brain size is the most
important determinant of the degree of basicranial flexion, whereas habitual head and
neck posture has no significant effect on
flexion. This result confirms the results obtained by Ross and Ravosa (1993) using
HEAD AND NECK POSTURE
more simplistic, qualitative descriptions of
posture.
Similarly, when head and neck posture is
taken into account, neither CBA nor IRE is
found to be significantly correlated with
AOA. Rather, when other variables are controlled for, variation in the degree of orbital
kyphosis (AOA) is significantly correlated
only with relative head and neck posture
(HNA). A factor that may contribute to this
correlation is the fact that the orbital axis
(used to calculate AOA) is probably roughly
perpendicular to the orbital plane (used to
calculate HNA) in most species. Thus, the
orientations of the axis and plane might be
expected to covary, with the consequence
that HNA and AOA would covary as well.
Ross and Ravosa (1993) found significant
partial correlations between CBA and both
AOA and IRE across primates and haplorhines and among colobines. Among platyrrhines, AOA was found to be more important
than IRE in determining CBA. Ross and
Ravosa’s (1993) results are clarified by the
present study: that is, among the animals
examined here—predominantly catarrhines—
flexion is predominantly determined by relative brain size and AOA is determined primarily by head and neck posture.
This latter finding corroborates a suggestion advanced elsewhere (Dabelow, 1929;
Ross, 1995) that high degrees of orbital
kyphosis (i.e., orbital margins that are ‘‘rotated’’ inferiorly relative to the rest of the
skull) might be expected in orthograde animals as one means of reorienting the visual
field. This explanation was invoked to explain the high degrees of orbital frontation
in indriids and callitrichids. The present
study is consistent with the hypothesis that
orthograde animals manifest vertically oriented orbital margins (equivalent to a ventrally deflected orbital axis and low value for
AOA) even when the degree of basicranial
flexion and relative brain size are taken into
account.
These results suggest that although the
relative size of the brain can mediate the
orientation of the upper face via basicranial
flexion, habitual head and neck posture have
an additional effect independent of flexion.
219
Implications for hominid evolution
Ross and Henneberg (1995) suggested that
although basicranial flexion in anthropoid
primates is primarily determined by relative brain size (Ross and Ravosa, 1993), the
hominid basicranium reached its maximal
degree of flexion early in hominid evolution,
possibly in Australopithecus africanus. This
maximal limit exists because the basicranium cannot flex to such an extent as to be in
danger of occluding the respiratory airway.
Subsequent to this stage of basicranial evolution, further increases in relative brain
size must be accommodated by other means.
The present study confirms the importance of relative brain size in determining
the degree of basicranial flexion among primates and suggests that head and neck
posture has little influence on the degree of
flexion among primates. However, it does
suggest that head and neck posture influences orbital axis orientation. This may
explain why anatomically modern humans
and the Kabwe skull exhibit orbital axes
that are more ventrally deflected than expected for their degree of basicranial flexion
(Ross and Henneberg, 1995). Rather than
being a mechanism for accommodating a
hypertrophied brain in the context of constraints on basicranial flexion (Ross and
Henneberg, 1995), a ventrally deflected orbital axis in humans may function instead to
bring the eyes and orbits into the correct,
anterior orientation.
Regarding head posture, the data indicate
that the orbital plane may approximate a
natural plane of head posture insofar as the
orbital plane exhibits a fairly consistent
relationship with gravity across diverse primates. The competing plane, Frankfurt Horizontal, is truly horizontal only in humans
and in all likelihood H. ergaster and later
hominids. Thus, the use of Frankfurt Horizontal as a registration plane still seems
justified in certain studies restricted to the
later half of human evolution (e.g., comparisons between archaic and anatomically modern humans). However, the Frankfurt plane
seems unsuited for studies that concern
early hominid evolution. This certainly is
true if the measurements being taken have
a functional relationship with posture or
220
D.S. STRAIT AND C.F. ROSS
nium attaches to the face. Obviously, there
is no clear functional reason why characters
such as these should be related to head
posture, so the justification for choosing the
orbital plane over the Frankfurt plane is not
as strong as it would be if a functional
relationship with posture existed. It is clear,
however, that without a reference to head
posture the Frankfurt plane is merely arbitrary, and it could be argued that data
collected against this plane are arbitrary as
well. Certainly it is disturbing that something as innocuous as how one holds a
cranium can have a substantial effect on
how one characterizes that cranium, and it
is a factor that needs to be considered more
critically in studies of early human evolution.
Fig. 6. Head posture and interpretations of early
hominid cranial morphology. A: KNM-WT 17000 oriented in Frankfurt Horizontal. B: KNM-WT 17000
oriented such that the orbital plane faces slightly inferiorly. C: OH 5 oriented in Frankfurt Horizontal. D: OH 5
oriented such that the orbital plane faces slightly inferiorly.
otherwise are meant to reflect the configuration of a structure relative to the position of
the head as it is held in habitual postures.
For such studies, the orbital plane provides
a much more suitable registration plane.
Even in a nonfunctional context, an acceptance of the orbital plane as a registration
plane has implications for how early hominid cranial morphology is interpreted. For
instance, Figure 6 depicts two fossil crania,
KNM-WT 17000 and OH 5, oriented in
Frankfurt Horizontal (Fig. 6A,C) and in
such a way as to make the orbits face
slightly inferiorly (Fig. 6B,D). In the Frankfurt plane, which is the standard view (e.g.,
Tobias, 1967; Walker et al., 1986; Walker
and Leakey, 1988), it seems obvious that the
two crania differ in facial orientation and
prognathism but are generally similar in
terms of facial hafting (i.e., the relationship
between the top of the face and the frontal
squama). However, when the skulls are oriented in a plane that is more likely to
represent habitual head posture, the differences in facial prognathism and orientation
seem less pronounced, whereas there is a
dramatic difference in how the neurocra-
CONCLUSIONS
Attempts to relate comparative cranial
morphology to posture within primates have
been limited by an absence of quantitative
kinematic data on head and neck posture.
With respect to basicranial flexion, data
presented here indicate that although the
premise of the postural hypothesis was confirmed (that primates maintain an anteriorly directed visual field), head and neck
posture is not the primary determinant of
flexion. Rather, flexion is influenced principally by relative brain size, a result that is
consistent with prior studies.
In a broader biological context, the kinematic data are relevant to any functional
study examining the relationship between
cranial morphology and posture not only
because information about head and neck
posture may be necessary to test those hypotheses but also because the head posture
data suggest that the orbital plane may be
suitable for use as a registration plane for
collecting craniometric measurements.
ACKNOWLEDGMENTS
The authors thank the following individuals and institutions: F.E. Grine, A.B. Demes,
W.L. Jungers, B.A. Wood, S. Larson, J.T.
Stern, S. DuMond, S. Jacques, Monkey
Jungle, S. Evans, the DuMond Conservancy,
F. Blanco, the Miami Metro Zoo, the Center
for Chimpanzee and Orangutan Conservation,
G. Eide, D. Lieberman, E.J.E. Szathmáry, and
HEAD AND NECK POSTURE
three anonymous reviewers. This research was
supported by an NSF doctoral dissertation
improvement grant (SBR9528921).
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